Until somewhat recently, optical fiber transmission systems were believed to be secure from intrusion. However, it is now known that a signal can be tapped from any length of optical fiber. A first step is for an intruder to remove portions of a cable sheath system and expose the optical fiber. Then some of the light transmitted through an optical fiber core is deflected to a receiver of the intruder. The extent to which signals in an unprotected optical fiber system can be detected depends on the intrinsic noise of the intruding receiver.
Now that it is known that optical fiber transmission systems are not so immune, there is a need to determine if and when intrusion occurs. In other words, a monitoring system is needed to provide an indication of the intrusion of optical fiber systems. Experts in the field have said that access to the optical fiber cables is not difficult and that there are few good ways of detecting intrusion unless the activity inside the fiber can be monitored.
Such monitoring or sensing systems are needed in an optical fiber system wherein sensitive information is transmitted. Aside from the obvious need to detect intrusion into optical fiber systems used in the military establishment, there also is such a need in civilian commercial enterprises such as in banking, for example.
Banking is one line of commerce where security against intrusion is needed as streams of financial data flow along an increasing number of optical fiber networks. Bank customers expect confidentiality and banks themselves need information control to compete. At the wholesale level where the largest money transactions take place, fiber tapping is not perceived to be a significant threat at this time. However, the movement of large sums of money in commerce obviously requires a high degree of security.
In an attempt to provide suitable security, the Federal Reserve System has evinced a desire to encrypt its telecommunications network by using data codes. A problem with encryption is how to manage safely the access to keys that unlock the data codes.
As for voice encryption, several problems are posed. One is the difficulty of preserving voice recognition once the message has been decoded. The other is the cost involved in providing individual encryption devices for each user. Furthermore, after a spy scandal surfaced recently, encryption no longer has been considered as the best protection for classified voice and data transmission.
The prior art includes systems intended to address the problem of security in optical fiber transmission systems. To protect cable, some advise installing the cable in a conduit and running a current along the conduit. An ohmmeter would detect any intrusion into the conduit. In a more sophisticated arrangement, a cable-tapping device draws off power through a microbend in the cable. The tapped power is run through a computer programmed to recognized disparities and sound an alarm and describe by way of a computer-screen menu the location of the intrusion. For such a system, the present cost is reported to be about $1000. per link.
Optical fiber can be used to detect a tap by means of optical fiber power loss that can be monitored by an optical time domain reflectometer or a sensitive photo detector. Typically, the prior art solutions have taken the course of monitoring the optical power transmission through each optical fiber. Should an optical fiber transmission system be intruded, an optical signal is tapped and optical power is extracted from the line. At the ends of the transmission line, detectors look for a change in optical power. If such a change is sensed, an alarm is set off. In such an arrangement, each optical fiber has to be monitored to be secured. Obviously, this solution is an expensive one.
More recently, in an article appearing in Volume 24, No. 13 of Electronics Letters dated June 23, 1988 and entitled "Integrated Communication and Sensing System Using One Single Mode Optical Fiber", the author discloses the use of an interferometric sensing technique to detect intrusion. This interferometric sensing technique utilizes the interference between different bound modes of an optical fiber that travel in the same direction along a fiber path, commonly referred to as a speckle pattern. However, to use this technique for single mode fiber systems where only one mode type exists, an additional wavelength, which is referred to as a monitoring wavelength and which is in the wavelength range of multimode operation of the fiber, must be introduced into the fiber path. This monitoring wavelength then launches more than one mode into the fiber. The consequent changes in speckle pattern which reflect changes in the phase relationship between these modes can be monitored. Whenever the optical fiber is touched, for example, that speckle pattern changes. A detector is positioned to be spaced from an end of the converted single mode fiber. When the special optical fiber is touched, the speckle pattern is displaced from the zone of pick up of the detector. In other words, the detector position can be varied to alter the degree of sensitivity.
Although this last described arrangement avoids the necessity for associating a monitor with each optical fiber in a cable, it does have certain drawbacks. For example, the monitoring wavelength that is used must be one at which the optical fiber operates multimode. This means that a relatively short wavelength must be used. However, as is well known, the loss in an optical fiber increases rapidly as the operating wavelength decreases because of Rayleigh scattering. For example, at a wavelength of 0.63 micron, the loss in the optical fiber is approximately eight to ten times as much as the loss at an operating wavelength of 1300 nanometers. As a result, this system cannot be used as a system other than one including a relatively short length of optical fiber. In fact, in the above-identified article appearing in Electronics Letters, mention is made that the system can work at distances out to several kilometers. Also, the last described arrangement is not as stable as desired inasmuch as the optical fiber can be moved relative to the detector. As a result, it is subject to vibrations, and false alarms of security violations may occur.
It should be clear that what is needed and what is not yet available is an optical fiber sensing system which is low in cost and which may be used to provide security for both voice and data transmission. What is desired is a system which does not need to be associated with each optical fiber in a cable. Hopefully, such a sought-after system would enjoy a wide spectrum of applicability in providing security in a number of environments.